High frequency heating apparatus with displacement identifiable remaining heating duration and phase control based thereon

ABSTRACT

In a high frequency heating apparatus, a microcomputer inputs the phase signal of a commercial AC power supply and determines whether a timer switch in a mechanical timer apparatus is in ON-state to determine whether the high frequency heating apparatus is in a heating-time set state. If so, the microcomputer turns on a switch that controls the conduction between the commercial AC power supply and a high voltage transformer at such a timing that minimizes the rush current in accordance with the phase of the commercial AC power supply, thereby suppressing the rush current, allowing the user to visually confirm the remaining heating time, and realizing a cost reduction.

TECHNICAL FIELD

The present invention relates to a high-frequency heating apparatus.More particularly, the present invention relates to a high-frequencyheating apparatus in which a heating duration is set by means of atime-limiting device including a mechanical switch (hereinafter referredto as a mechanical time-limiting device).

BACKGROUND ART

As an example of a conventional high-frequency heating apparatus, amicrowave oven will be described below. An example of the configurationof a conventional microwave oven is shown in FIG. 6. A mechanicaltime-limiting device 1″ has the following components assembled into aunit: a heating duration setter 1 a, a bell 1 b, a mechanical switchfunctioning as a time-limiting switch 1 e, and a mechanical switchfunctioning as a time-limiting switch 1 f. One end of commercialalternating-current power source 2 is connected via the time-limitingswitch 1 e to one end of the time-limiting switch 1 f. The other end ofthe time-limiting switch 1 f is connected via a surge circuit 10 to oneend of a primary coil of a high-voltage transformer 5. On the otherhand, the other end of the commercial alternating-current power source 2is connected directly to the other end of the primary coil of thehigh-voltage transformer 5.

The surge circuit 10 is composed of a surge input monitoring circuit 11,a switch 12, and a resistor R2. One end of the surge input monitoringcircuit 11, one end of the switch 12, and one end of the resistor R2 areconnected to the time-limiting switch 1 f. The other end of the switch12 and the other end of the resistor R2 are connected to the primarycoil of the high-voltage transformer 5. The other end of the surge inputmonitoring circuit 11 is connected to the node between the other end ofthe commercial alternating-current power source 2 and the other end ofthe primary coil of the high-voltage transformer 5.

Moreover, one end of electric circuitry 3 (hereinafter referred to asthe high-frequency heating oscillator cooling device and othercomponents 3) including components—such as an oven lamp for illuminatingthe interior of a heating chamber, a turntable motor for rotating aturntable, and a fan motor for cooling a magnetron 6—that need to beoperated as high-frequency heating is performed is connected to the nodebetween the time-limiting switches 1 e and 1 f. Moreover, the other endof the high-frequency heating oscillator cooling device and othercomponents 3 is connected to the node between the other end of thecommercial alternating-current power source 2 and to the other end ofthe primary coil of the high-voltage transformer 5.

Next, the components connected to the secondary side of the high-voltagetransformer will be described. Between the anode and cathode of themagnetron 6 is connected a diode D1 in parallel therewith. Specifically,to the anode of the magnetron 6 is connected the cathode of the diodeD1, and to the cathode of the magnetron 6 is connected the anode of thediode D1. Moreover, to the cathode of the magnetron 6 is connected asecondary coil 5 a of the high-voltage transformer 5. Furthermore, tothe anode of the diode D1 is connected, via a capacitor C1, one end of asecondary coil 5 b of the high-voltage transformer 5, and to the cathodeof the diode D1 is connected the other end of the secondary coil 5 b.The anode of the magnetron 6 is grounded.

Now, the operation of the microwave oven configured as described abovewill be described. The heating duration setter 1 a has a rotary knob(not illustrated). When the user rotates the rotary knob clockwise, theheating duration setter 1 a sets a heating duration commensurate withthe amount of rotation. As the heating duration passes by, the rotaryknob rotates counter-clockwise by a rotation angle commensurate with thelapsed time, thereby indicating the remaining heating duration on ananalog basis. The time-limiting switch 1 e remains on during the heatingduration, and otherwise remains off. On the other hand, thetime-limiting switch 1 f, during the heating duration, toggles betweenon and off with a duty factor determined by the motor and gear-and-cammechanism (not illustrated) incorporated in the heating duration setter1 a, and otherwise remains off. At the end of the heating duration, thebell 1 b sounds.

When the time-limiting switch 1 e is on, i.e. during the heatingduration, electric power is supplied from the commercialalternating-current power source 2 to the high-frequency heatingoscillator cooling device and other components 3, so that thehigh-frequency heating oscillator cooling device and other components 3operate.

When the time-limiting switch 1 e is on and the time-limiting switch 1 fis on, electric power is supplied from the commercialalternating-current power source 2 to the high-voltage transformer 5, sothat a high voltage of about 4 kV appears at the secondary side of thehigh-voltage transformer 5. This high voltage is supplied to themagnetron 6, so that the magnetron 6 oscillates a microwave. A target tobe heated is irradiated with this microwave, and is thereby heated.Here, through the time-limiting switches 1 e and 1 f flows a currentthat is needed to achieve microwave heating, and therefore thetime-limiting switches 1 e and 1 f need to be mechanical switchesthrough which a current of at least 15 A can be passed. On the otherhand, when the time-limiting switch 1 e is on and the time-limitingswitch 1 f is off, no electric power is supplied from the commercialalternating-current power source 2 to the high-voltage transformer 5, sothat the magnetron 6 oscillates no microwave. Thus, the microwave outputis determined by the duty factor mentioned above.

If the timing with which the time-limiting switch 1 f turns from off toon is not in synchronism with the phase of the commercialalternating-current power source 2, the exciting current of thehigh-voltage transformer 5 may produce a large rush current, over 100 Ain the worst case. It is for this reason that, in the conventionalmicrowave oven shown in FIG. 6, which cannot bring the timing with whichthe time-limiting switch 1 f turns from off to on into synchronism withthe phase of the commercial alternating-current power source 2, thesurge circuit 10 is provided with a view to suppressing rush current.

The switch 12 is controlled by the surge input monitoring circuit 11 soas to be normally on, short-circuiting the resistor R2. The surge inputmonitoring circuit 11 monitors the value of the rush current, and, whenthe monitored rush current becomes higher than a threshold value, keepsthe switch 12 off for a predetermined period. When the switch 12 is off,the rush current is branched via the resistor R2, reducing the effectthereof.

It is true that providing the surge circuit 10 as described above helpsreduce rush current. However, even the surge circuit 10 cannot minimizerush current, and thus a heavy burden remains imposed on thetime-limiting switch 1 f, through which a large rush current flows asusual when it turns from off to on.

Moreover, the surge circuit 10 is composed of rather large components.Thus, even in a microwave oven provided with a circuit board on which tomount electric components, unlike the other components mounted thereon,the surge circuit 10 is not mounted on the circuit board, but is fittedto the main unit of the microwave oven. This necessitates an extra stepof fitting the surge circuit in the manufacturing procedure, and thushinders cost reduction. Moreover, the large components of the surgecircuit 10 hinders size reduction.

On the other hand, there have conventionally been known alsohigh-frequency heating apparatuses, for example the one disclosed inJapanese Patent Application Laid-Open No. S63-205088, in which a switchfor controlling the supply of electric power to a high-voltagetransformer is controlled by a microcomputer so as to reduce rushcurrent. However, this type of high-frequency heating apparatus is notprovided with a mechanical time-limiting device that permits the user tovisually recognize the remaining heating duration in the form of theamount of rotation of a rotary knob. Thus, to permit the user tovisually recognize the remaining heating duration, it is necessary toadditionally provide a display device such as a liquid crystal display.Additionally providing such a display device leads to higher cost.

DISCLOSURE OF THE INVENTION

In view of the above problems, it is an object of the present inventionto provide an inexpensive high-frequency heating apparatus that canreduce rush current and that permits the user to visually recognize theremaining heating duration.

To achieve the above object, in one aspect of the present invention, ahigh-frequency heating apparatus is provided with: a high-frequencyoscillator; a high-voltage transformer that supplies electric powerappearing at the secondary side thereof to the high-frequencyoscillator; switching means that supplies electric power to the primaryside of the high-voltage transformer; a time-limiting device includingheating duration setting means that permits a heating duration to be setaccording to the amount of displacement by which the heating durationsetting means is displaced progressively and that is displacedretrogressively as the heating duration passes by, and a time-limitingswitch that turns from off to on when the heating duration is set andthat turns from on to off when the amount of displacement returns to theinitial value; phase monitoring means that monitors the phase of thesupply voltage; and controlling means that is fed with electric powerwhen the time-limiting switch is on and that controls the switchingmeans according to the phase of the supply voltage as monitored by thephase monitoring means.

With this configuration, the provision of the controlling means thatcontrols the switching means according to the phase of the suppliedvoltage helps reduce rush current. This permits the switching means tohave a lower rated current capacity, and thus helps achieve lower cost.Moreover, the time-limiting device permits the remaining heatingduration to be indicated on an analog basis, and thus eliminates theneed to provide a display device. In this way, it is possible to realizean inexpensive high-frequency heating apparatus that permits the user tovisually recognize the remaining heating duration. Furthermore, electricpower is supplied to the controlling means when the time-limiting switchis on, and no electric power is supplied to the controlling means whenthe time-limiting switch is off. This helps eliminate the stand-by powerconsumption of the controlling means.

Advisably, when the controlling means judges the time-limiting switch tobe off with respect to the value of the voltage supplied when thetime-limiting switch is on, the controlling means turns off theswitching means according to the phase of the supply voltage asmonitored by the phase monitoring means.

Thus, it is possible, without the provision of separate detecting meansfor detecting whether or not the time-limiting switch is on, to preventelectric discharge from occurring at the contacts of the switching meanswhen the switching means turns from on to off to stop the supply ofelectric power to the high-frequency oscillator.

Alternatively, detecting means that detects whether or not thetime-limiting switch is on is additionally provided, and, when thecontrolling means judges the time-limiting switch to be off according tothe result of detection by the detecting means, the controlling meansturns off the switching means according to the phase of the supplyvoltage as monitored by the phase monitoring means.

Thus, it is possible to obtain the same benefits as described above.

To achieve the above object, in another aspect of the present invention,a high-frequency heating apparatus is provided with: a high-frequencyoscillator; a high-voltage transformer that supplies electric powerappearing at the secondary side thereof to the high-frequencyoscillator; a time-limiting device including heating duration settingmeans that permits a heating duration to be set according to the amountof displacement by which the heating duration setting means is displacedand that indicates the remaining heating duration by letting the amountof displacement vary as the heating duration passes by, the amount ofdisplacement returning to the initial value when the set heatingduration has elapsed, and a time-limiting switch that turns on or offwhen the heating duration is set; switching means that controls thesupply of a supply voltage to the primary side of the high-voltagetransformer; and controlling means including means for monitoring thephase of the supply voltage and means for judging whether or not aheating duration is set or not by detecting whether the time-limitingswitch is on or off, the controlling means, if a heating duration isset, turning on the switching means according to the phase of the supplyvoltage.

With this configuration, the provision of the controlling means thatturns on the switching means according to the phase of the suppliedvoltage helps reduce rush current. This permits the switching means tohave a lower rated current capacity, and thus helps achieve lower cost.Moreover, the time-limiting device permits the remaining heatingduration to be indicated on an analog basis, and thus eliminates theneed to provide a display device. In this way, it is possible to realizean inexpensive high-frequency heating apparatus that permits the user tovisually recognize the remaining heating duration.

In either of the high-frequency heating apparatuses described above,advisably, when the absolute value of the supply voltage is at themaximum, the controlling means turns the switching means from off to on,and, when the supply voltage crosses the zero-Volt level, thecontrolling means turns the switching means from on to off.

Thus, the controlling means turns the switching means from off to onwhen the absolute value of the supply voltage is at the maximum, andthis helps reduce rush current. Moreover, the controlling means turnsthe switching means from on to off when the supply voltage crosses thezero-Volt level, and this prevents electric discharge from occurring atthe contacts of the switching means when it turns from on to off,greatly enhancing the durability of the switching means.

In either of the high-frequency heating apparatuses described above,advisably, the controlling means turns the switching means on and offwith a predetermined duty factor. This makes it possible to producedesired high-frequency output.

In either of the high-frequency heating apparatuses described above,advisably, high-frequency heating output setting means that sets thehigh-frequency heating output is additionally provided, and thecontrolling means varies the duty factor according to the output signalof the high-frequency heating output setting means. This makes itpossible to adjust the high-frequency heating output.

In either of the high-frequency heating apparatuses described above,advisably, the time-limiting switch only controls the supply of electricpower to the electric circuit that needs to operate when high-frequencyheating is performed.

With this configuration, the time-limiting switch only controls thesupply of electric power to the electric circuit that needs to operatewhen high-frequency heating is performed. Thus, the current that issupplied to the high-voltage transformer does not flow through thetime-limiting switch. This permits the time-limiting switch to berealized with a mechanical switch having a low rated current capacity,and thus helps achieve lower cost.

In either of the high-frequency heating apparatuses described above,advisably, the time-limiting device further includes a secondtime-limiting switch that, at the end of the heating duration, turnsfrom on to off with a delay relative to the time-limiting switch, andthe second time-limiting switch is connected in series with theswitching means.

With this configuration, the time-limiting device further includes thesecond time-limiting switch that, at the end of the heating duration,turns from on to off with a delay relative to the time-limiting switch,and the second time-limiting switch is connected in series with theswitching means. Thus, even if the controlling means becomes faulty, itis possible to end microwave heating. This helps enhance safety.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the configuration of the microwave oven ofthe first embodiment of the invention;

FIG. 2 is a diagram showing the configuration of the microwave oven ofthe second embodiment of the invention;

FIG. 3 is a diagram showing the configuration of the microwave oven ofthe third embodiment of the invention;

FIG. 4 is a diagram showing the configuration of the microwave oven ofthe fourth embodiment of the invention;

FIG. 5 is a diagram showing the configuration of the microwave oven ofthe fifth embodiment of the invention; and

FIG. 6 is a diagram showing the configuration of a conventionalmicrowave oven.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. In the following description,microwave ovens will be taken up as examples of high-frequency heatingapparatuses according to the invention.

First Embodiment

The configuration of the microwave oven of the first embodiment of theinvention is shown in FIG. 1. Here, such components as are found also inthe microwave oven shown in FIG. 6 are identified with common referencenumerals.

A mechanical time-limiting device 1 has the following componentsassembled into a unit: a heating duration setter 1 a, a bell 1 b, and amechanical switch functioning as a time-limiting switch 1 c.

One end of commercial alternating-current power source 2 is connectedvia a relay switch 4 a to one end of a primary coil of a high-voltagetransformer 5. On the other hand, the other end of the commercialalternating-current power source 2 is connected directly to the otherend of the primary coil of the high-voltage transformer 5.

One end of high-frequency heating oscillator cooling device and othercomponents 3 is connected to the node between the commercialalternating-current power source 2 and the relay switch 4 a, and theother end of the high-frequency heating oscillator cooling device andother components 3 is connected via the time-limiting switch 1 c to thenode between the other end of the commercial alternating-current powersource 2 and the other end of the primary coil of the high-voltagetransformer 5. Moreover, the input side of a phase signal circuit 7 isconnected directly to both ends of the commercial alternating-currentpower source 2, and the output side of the phase signal circuit 7 isconnected to a microcomputer 9.

Furthermore, the input side of a rectification, smoothing, andvoltage-division circuit 8 is connected to the node between thetime-limiting switch 1 c and the high-frequency heating oscillatorcooling device and other components 3, and the output side of therectification, smoothing, and voltage-division circuit 8 is connected tothe microcomputer 9. The microcomputer 9 is connected to a drive circuit4 b for driving the relay switch 4 a.

The components that are connected to the secondary side of thehigh-voltage transformer 5 are the same as in the microwave oven shownin FIG. 6, and therefore their explanations will be omitted.

Now, the operation of the microwave oven configured as described abovewill be described. The heating duration setter 1 a has a rotary knob(not illustrated). When the user rotates the rotary knob clockwise, theheating duration setter 1 a sets a heating duration commensurate withthe amount of rotation. As the heating duration passes by, the rotaryknob rotates counter-clockwise by a rotation angle commensurate with thelapsed time, thereby indicating the remaining heating duration on ananalog basis. The time-limiting switch 1 c remains on during the heatingduration, and otherwise remains off. At the end of the heating duration,the bell 1 b sounds.

The phase signal circuit 7 receives from the commercialalternating-current power source 2 an alternating-current voltage,produces a phase signal that represents the phase of thealternating-current voltage, and feeds the thus-produced phase signal tothe microcomputer 9.

When the time-limiting switch 1 c is on, i.e. during the heatingduration, alternating-current electric power is supplied from thecommercial alternating-current power source 2 to the high-frequencyheating oscillator cooling device and other components 3, so that thehigh-frequency heating oscillator cooling device and other components 3operate. Moreover, when the time-limiting switch 1 c is on, i.e. duringthe heating duration, an alternating-current voltage is supplied fromthe commercial alternating-current power source 2 to the rectification,smoothing, and voltage-division circuit 8. The rectification, smoothing,and voltage-division circuit 8 includes a rectification diode, asmoothing capacitor, and voltage-division resistors, and, when suppliedwith the alternating-current voltage, feeds a direct-voltage signalhaving a predetermined value to the microcomputer 9, which functions asa controlling means.

The microcomputer 9 is supplied with stabilized direct-current electricpower from a regulator (not illustrated), and is kept energized all thetime.

The microcomputer 9 judges whether a heating duration is set or notaccording to the direct-current signal output from the rectification,smoothing, and voltage-division circuit 8. When a heating duration isrecognized to be set, the microcomputer 9 turns the relay switch 4 afrom off to on at the moment when the phase of the output voltage of thecommercial alternating-current power source 2 is such that rush currentis minimal (i.e. the phase is such that the absolute value of the outputvoltage is maximal). The microcomputer 9 does this according to thephase signal output from the phase signal circuit 7, and taking intoconsideration the delay that accompanies the operation of the drivecircuit 4 b for driving the relay switch 4 a. When the set heatingduration has elapsed, the microcomputer 9 turns the relay switch 4 afrom on to off at the moment when the output voltage of the commercialalternating-current power source 2 crosses the zero-Volt level, andholds the relay switch 4 a off. The microcomputer 9 does this accordingto the phase signal output from the phase signal circuit 7, and takinginto consideration the delay that accompanies the operation of the drivecircuit 4 b for driving the relay switch 4 a. All the while in between,the microcomputer 9 keeps turning the relay switch 4 a on and off with apredetermined duty factor.

When the relay switch 4 a is on, electric power is supplied from thecommercial alternating-current power source 2 to the high-voltagetransformer 5, and thus a high voltage of about 4 kV appears at thesecondary side of the high-voltage transformer 5. This high voltage issupplied to a magnetron 6, so that the magnetron 6 oscillates amicrowave. A target to be heated is irradiated with this microwave, andis thereby heated. Here, as described above, the relay switch 4 a turnsfrom off to on at the moment when rush current is minimal, and thishelps greatly enhance the durability of the relay switch 4 a. On theother hand, when the relay switch 4 a is off, no electric power issupplied from the commercial alternating-current power source 2 to thehigh-voltage transformer 5, so that the magnetron 6 oscillates nomicrowave. Thus, the microwave output is determined by the duty factormentioned above. Here, as described above, the relay switch 4 a turnsfrom on to off at the moment when the output voltage of the commercialalternating-current power source 2 crosses the zero-Volt level, and thisprevents electric discharge from occurring at the contacts of the relayswitch 4 a, greatly enhancing the durability of the relay switch 4 a.

Moreover, the current that is supplied to the high-voltage transformer 5does not flow through the time-limiting switch 1 c; therefore, only asmall current flows through the time-limiting switch 1 c. This makes itpossible to realize the time-limiting switch 1 c with a mechanicalswitch having a low rated current capacity, and thus helps achieve lowercost.

Moreover, smaller rush current makes it possible to realize the relayswitch 4 a with a switch having a lower rated current capacity than theconventionally used time-limiting switches 1 e and 1 f (see FIG. 6).This helps achieve lower cost.

Moreover, it is possible, without the use of a display device, to makethe mechanical time-limiting device 1 indicate the remaining heatingduration. This makes it possible to realize an inexpensive microwaveoven that permits the user to visually recognize the remaining heatingduration.

Alternatively, the microcomputer 9 may start measuring time with a timerincorporated therein at the start of heating in order to monitor thelapse of a predetermined period so that, after the timer has recognizedthe lapse of the predetermined period, the microcomputer 9 keeps therelay switch 4 a off irrespective of whether or not the direct-currentvoltage signal from the rectification, smoothing, and voltage-divisioncircuit 8 is present. In this case, advisably, the predetermined periodmentioned above is set to be longer than the maximum value of theheating duration that can be set on the heating duration setter 1 a. Byso doing, even if the mechanical time-limiting device 1 becomes faultyand leaves the time-limiting switch 1 c on even after the end of theheating duration, the microcomputer 9, after the lapse of thepredetermined period, can turn the relay switch 4 a off and endmicrowave heating.

Second Embodiment

The configuration of the microwave oven of the second embodiment of theinvention is shown in FIG. 2. Here, such components as are found also inthe microwave oven shown in FIG. 1 are identified with common referencenumerals.

A mechanical time-limiting device 1′ has the following componentsassembled into a unit: a heating duration setter 1 a, a bell 1 b, amechanical switch functioning as a time-limiting switch 1 c′, and amechanical switch functioning as a second time-limiting switch 1 d.

One end of commercial alternating-current power source 2 is connected,via the second time-limiting switch 1 d and a relay switch 4 a in thisorder, to one end of a primary coil of a high-voltage transformer 5. Onthe other hand, the other end of the commercial alternating-currentpower source 2 is connected directly to the other end of the primarycoil of the high-voltage transformer 5.

One end of high-frequency heating oscillator cooling device and othercomponents 3 is connected to the node between the second time-limitingswitch 1 d and the relay switch 4 a, and the other end of thehigh-frequency heating oscillator cooling device and other components 3is connected to the node between the other end of the commercialalternating-current power source 2 and the other end of the primary coilof the high-voltage transformer 5. Moreover, the input side of a phasesignal circuit 7 is connected directly to both ends of the commercialalternating-current power source 2, and the output side of the phasesignal circuit 7 is connected to a microcomputer 9.

Furthermore, both ends of the time-limiting switch 1 c′ are connected tothe microcomputer 9. The microcomputer 9 is connected to a drive circuit4 b for driving the relay switch 4 a.

The components that are connected to the secondary side of thehigh-voltage transformer 5 are the same as in the microwave ovens shownin FIGS. 1 and 6, and therefore their explanations will be omitted.

Now, the operation of the microwave oven configured as described abovewill be described. The heating duration setter 1 a has a rotary knob(not illustrated). When the user rotates the rotary knob clockwise, theheating duration setter 1 a sets a heating duration commensurate withthe amount of rotation. As the heating duration passes by, the rotaryknob rotates counter-clockwise by a rotation angle commensurate with thelapsed time, thereby indicating the remaining heating duration on ananalog basis. The time-limiting switch 1 c′ and the second time-limitingswitch 1 d remain on when a heating duration is set, and otherwiseremain off. Here, the second time-limiting switch 1 d is a mechanicalswitch that, at the end of the heating duration, turns from on to offwith a predetermined length of delay (several seconds) relative to thetiming with which the time-limiting switch 1 c′ turns from on to off.This can be achieved by differentiating the shape of the cam thatoperates the time-limiting switch 1 c′ from the shape of the cam thatoperates the second time-limiting switch 1 d. At the end of the heatingduration, the bell 1 b sounds. The time-limiting switch 1 c′ may be ofthe type that remains off when a heating duration is set.

The phase signal circuit 7 receives from the commercialalternating-current power source 2 an alternating-current voltage,produces a phase signal that represents the phase of thealternating-current voltage, and feeds the thus produced phase signal tothe microcomputer 9.

When the second time-limiting switch 1 d is on, i.e. during the heatingduration, alternating-current electric power is supplied from thecommercial alternating-current power source 2 to the high-frequencyheating oscillator cooling device and other components 3, so that thehigh-frequency heating oscillator cooling device and other components 3operate. Moreover, when the time-limiting switch 1 c′ is on, i.e. duringthe heating duration, a short-circuit signal is fed via thetime-limiting switch 1 c′ to the microcomputer 9.

The microcomputer 9 is supplied with stabilized direct-current electricpower from a regulator (not illustrated), and is kept energized all thetime.

The microcomputer 9 judges whether a heating duration is set or notaccording to the short-circuit signal generated when the time-limitingswitch 1 c′ turns on.

When a heating duration is recognized to be set, the microcomputer 9turns the relay switch 4 a from off to on at the moment when the phaseof the output voltage of the commercial alternating-current power source2 is such that rush current is minimal (i.e. the phase is such that theabsolute value of the output voltage is maximal). The microcomputer 9does this according to the phase signal output from the phase signalcircuit 7, and taking into consideration the delay that accompanies theoperation of the drive circuit 4 b for driving the relay switch 4 a.

At the end of the set heating duration, the microcomputer 9 turns therelay switch 4 a from on to off at the moment when the output voltage ofthe commercial alternating-current power source 2 crosses the zero-Voltlevel, and holds the relay switch 4 a off. The microcomputer 9 does thisaccording to the phase signal output from the phase signal circuit 7,and taking into consideration the delay that accompanies the operationof the drive circuit (not illustrated) for driving the switch 4. Then,as described above, after the end of the heating duration, the secondtime-limiting switch 1 d turns off with the predetermined length ofdelay (several seconds) relative to the time-limiting switch 1 c′. Thislength of delay is set to be sufficiently long to permit the relayswitch 4 a to be held off after the end of the heating duration. Thus,after the end of the heating duration, first the relay switch 4 a turnsoff, and thereafter the second time-limiting switch 1 d turns off. Thisprevents electric discharge from occurring at the contacts of the secondtime-limiting switch 1 d when it turns from on to off, and thus helpsgreatly enhance the durability of the second time-limiting switch 1 d.

Alternatively, the microcomputer 9 may start measuring time with a timerincorporated therein at the start of heating in order to monitor thelapse of a predetermined period so that, after the timer has recognizedthe lapse of the predetermined period, the microcomputer 9 keeps therelay switch 4 a off irrespective of whether or not the short-circuitsignal is present. In this case, advisably, the predetermined periodmentioned above is set to be longer than the maximum value of theheating duration that can be set on the heating duration setter 1 a. Byso doing, even if the mechanical time-limiting device 1 becomes faultyand leaves the time-limiting switch 1 c′ and the second time-limitingswitch 1 d on even after the end of the heating duration, themicrocomputer 9, after the lapse of the predetermined period, can turnthe relay switch 4 a off and end microwave heating.

Moreover, even if the microcomputer 9 becomes faulty and leaves therelay switch 4 a on even after the end of the heating duration, or ifthe relay switch 4 a becomes fused, the mechanical time-limiting device1′ can, after the end of the heating duration, turn the time-limitingswitch 1 d off and end microwave heating.

In this way, in the microwave oven shown in FIG. 2, even if either ofthe microcomputer 9 and the mechanical time-limiting device 1′ becomesfaulty, microwave heating can be ended. This helps enhance safety.

The microwave oven shown in FIG. 2, as compared with the microwave ovenshown in FIG. 1, requires the additional provision of the secondtime-limiting switch 1 d but does not require the rectification,smoothing, and voltage-division circuit 8. Thus, in terms of cost, themicrowave oven of FIG. 2 roughly compares with that of FIG. 1.

Third Embodiment

The configuration of the microwave oven of the third embodiment of theinvention is shown in FIG. 3. Here, such components as are found also inthe microwave oven shown in FIG. 1 are identified with common referencenumerals.

The microwave oven of the third embodiment, as compared with themicrowave oven of the first embodiment described earlier, isadditionally provided with a variable-contact resistor R1. One end ofthe variable-contact resistor R1 is connected to one end of thecommercial alternating-current power source 2, and the other end of thevariable-contact resistor R1 is connected to the other end of thecommercial alternating-current power source 2. The movable contact ofthe variable-contact resistor R1 is connected via a rectification,smoothing, and voltage-division circuit 8′ to the microcomputer 9. Theuser can operate the variable-contact resistor R1 to change the positionof the movable contact thereof. The alternating-current voltage at themovable contact of the variable-contact resistor R1 is converted into adirect-current voltage by the rectification, smoothing, andvoltage-division circuit 8′, and is then fed to the microcomputer 9.According to this direct-current voltage, the microcomputer 9 varies theduty factor with which it turns the relay switch 4 a on and off. Thismakes it possible to adjust the microwave output.

Fourth Embodiment

The configuration of the microwave oven of the fourth embodiment of theinvention is shown in FIG. 4. Here, such components as are found also inthe microwave oven shown in FIG. 1 are identified with common referencenumerals.

Whereas, in the microwave oven shown in FIG. 1, the microcomputer 9 iskept energized all the time, in the microwave oven shown in FIG. 4, themicrocomputer 9 is energized only when a heating duration is set on themechanical time-limiting device 1. This is the most significantdifference between these microwave ovens. To achieve this, therectification, smoothing, and voltage-division circuit 8 used in themicrowave oven shown in FIG. 1 is here replaced with a microcomputerpower supply circuit 13, which corresponds to what is referred to as theregulator (not illustrated) in the microwave oven shown in FIG. 1. Oneinput of the microcomputer power supply circuit 13 is connected to thenode between the commercial alternating-current power source 2 and therelay switch 4 a, and another input of the microcomputer power supplycircuit 13 is connected via the time-limiting switch 1 c to the nodebetween the other end of the commercial alternating-current power source2 and the other end of the primary coil of the high-voltage transformer5. The output side of the microcomputer power supply circuit 13 isconnected to the microcomputer 9.

Now, an outline of the operation of the microwave oven configured asdescribed above will be described, with emphasis placed on differencesfrom the microwave oven shown in FIG. 1. When a heating duration is set,the time-limiting switch 1 c turns on, and thus an alternating-currentvoltage is supplied from the commercial alternating-current power source2 to the high-frequency heating oscillator cooling device and othercomponents 3 and to the microcomputer power supply circuit 13. Whensupplied with the alternating-current voltage from the commercialalternating-current power source 2, the microcomputer power supplycircuit 13 supplies direct-current electric power to the microcomputer9, so that the microcomputer 9 is energized.

When the electric power supplied to the microcomputer 9 becomes higherthan the operating voltage thereof, the microcomputer 9 is reset, andstarts executing the program incorporated therein. Specifically, themicrocomputer 9 turns the relay switch 4 a from off to on at the momentwhen the phase of the output voltage of the commercialalternating-current power source 2 is such that the rush current isminimal (i.e. the phase is such that the absolute value of the outputvoltage is maximal). The microcomputer 9 does this according to thephase signal output from the phase signal circuit 7, and taking intoconsideration the delay that accompanies the operation of the relayswitch drive circuit 4 b.

At the end of the set heating duration, the time-limiting switch 1 cturns off. When the time-limiting switch 1 c turns off, the outputvoltage of the microcomputer power supply circuit 13 gradually decreasesuntil it ultimately becomes zero. When the output voltage of themicrocomputer power supply circuit 13 becomes lower than a previouslyset threshold value (for example, equal to 80% of the output voltageobtained when the time-limiting switch 1 c is on), the microcomputer 9recognizes the end of the heating duration, and thus turns the relayswitch 4 a from on to off at the moment when the output voltage of thecommercial alternating-current power source 2 crosses the zero-Voltlevel, and holds the relay switch 4 a off. The microcomputer 9 does thisaccording to the phase signal output from the phase signal circuit 7,and taking into consideration the delay that accompanies the operationof the relay switch drive circuit 4 b. Thereafter, when the outputvoltage of the microcomputer power supply circuit 13 becomes lower thanthe minimum operating voltage of the microcomputer 9, the microcomputer9 stops its operation.

When the relay switch 4 a is on, electric power is supplied from thecommercial alternating-current power source 2 to the high-voltagetransformer 5, and thus a high voltage appears at the secondary side ofthe high-voltage transformer 5. This high voltage is supplied to themagnetron 6, so that the magnetron 6 oscillates a microwave. A target tobe heated is irradiated with this microwave, and is thereby heated.

In the microwave oven shown in FIG. 4, the microcomputer 9 is not keptenergized all the time. This helps save electric power. In addition,quite naturally, the microwave oven shown in FIG. 4 gives the samebenefits as the microwave oven shown in FIG. 1. In the microwave ovenshown in FIG. 4, the microcomputer 9 refers to the output voltage of themicrocomputer power supply circuit 13 to detect whether or not thetime-limiting switch 1 c is off to recognize the end of the heatingduration. Alternatively, it is also possible, for example, to provide aseparate rectification, smoothing, voltage-division circuit thatreceives the voltage at the node between the time-limiting switch 1 cand the microcomputer power supply circuit 13, that then rectifies,smooths, and divides that voltage to convert it into a direct-currentvoltage, and that then feeds the resulting direct-current voltage to themicrocomputer 9. In this case, according to the output of thisrectification, smoothing, voltage-division circuit, the microcomputer 9detects whether or not the time-limiting switch 1 c is off to recognizethe end of the heating duration.

Fifth Embodiment

FIG. 5 shows, as another feasible example, the microwave oven of thefifth embodiment of the invention. Here, the time-limiting switch 1 c isprovided between the commercial alternating-current power source 2 andthe node among the high-frequency heating oscillator cooling device andother components 3, high-voltage transformer 5, and microcomputer powersupply circuit 13. That is, the relay switch 4 a is not controlled atthe end of the heating duration.

The microcomputer 9 in the microwave oven shown in FIG. 5 is not keptenergized all the time, in the same way as in the microwave oven shownin FIG. 4. This helps save electric power.

INDUSTRIAL APPLICABILITY

High-frequency heating apparatuses according to the present inventionshall find applications in various kinds of apparatuses capable ofhigh-frequency heating, including but not limited to microwave ovens.

1. A high-frequency heating apparatus comprising: a high-frequencyoscillator; a high-voltage transformer that supplies electric powerappearing at a secondary side thereof to the high-frequency oscillator;switching means that switches whether to supply or not to supplyelectric power to a primary side of the high-voltage transformer; atime-limiting device including heating duration setting means thatpermits a heating duration to be set according to an amount ofdisplacement by which the heating duration setting means is displacedforward and that is displaced backward as the heating duration passesby, and a time-limiting switch that turns from off to on when theheating duration is set and that turns from on to off when the amount ofdisplacement returns to an initial value; phase monitoring means thatmonitors a phase of the supply voltage; and controlling means that isfed with electric power when the time-limiting switch is on and thatcontrols the switching means according to the phase of the supplyvoltage as monitored by the phase monitoring means.
 2. Thehigh-frequency heating apparatus of claim 1, wherein, when thecontrolling means judges the time-limiting switch to be off according toa value of a voltage supplied when the time-limiting switch is on, thecontrolling means turns off the switching means according to the phaseof the supply voltage as monitored by the phase monitoring means.
 3. Thehigh-frequency heating apparatus of claim 1, further comprising:detecting means that detects whether or not the time-limiting switch isoff, wherein, when the controlling means judges the time-limiting switchto be off according to a result of detection by the detecting means, thecontrolling means turns off the switching means according to the phaseof the supply voltage as monitored by the phase monitoring means.
 4. Thehigh-frequency heating apparatus of claim 1, wherein, when an absolutevalue of the supply voltage is at a maximum, the controlling means turnsthe switching means from off to on, and, when the supply voltage crossesa zero-Volt level, the controlling means turns the switching means fromon to off.
 5. The high-frequency heating apparatus of claim 4, furthercomprising: high-frequency heating output setting means that setshigh-frequency heating output, wherein the controlling means turns theswitching means on and off with a duty factor according to an outputsignal of the high-frequency heating output setting means, wherein thetime-limiting switch only controls supply of electric power to anelectric circuit that needs to operate as high-frequency heating isperformed, wherein the time-limiting device further includes a secondtime-limiting switch that, at an end of the heating duration, turns fromon to off with a delay relative to the time-limiting switch, and whereinthe second time-limiting switch is connected in series with theswitching means.
 6. The high-frequency heating apparatus of claim 1,wherein the controlling means turns the switching means on and off witha predetermined duty factor.
 7. The high-frequency heating apparatus ofclaim 6, further comprising: high-frequency heating output setting meansthat sets high-frequency heating output, wherein the controlling meansvaries the duty factor according to an output signal of thehigh-frequency heating output setting means.
 8. The high-frequencyheating apparatus of claim 1 wherein the time-limiting switch onlycontrols supply of electric power to an electric circuit that needs tooperate as high-frequency heating is performed.
 9. The high-frequencyheating apparatus of claim 1, wherein the time-limiting device furtherincludes a second time-limiting switch that, at an end of the heatingduration, turns from on to off with a delay relative to thetime-limiting switch, and wherein the second time-limiting switch isconnected in series with the switching means.
 10. A high-frequencyheating apparatus comprising: a high-frequency oscillator; ahigh-voltage transformer that supplies electric power appearing at asecondary side thereof to the high-frequency oscillator; a time-limitingdevice including heating duration setting means that permits a heatingduration to be set according to an amount of displacement by which theheating duration selling means is displaced and that indicates aremaining heating duration by letting the amount of displacement vary asthe heating duration passes by, the amount of displacement returning toan initial value when the set heating duration has elapsed, and atime-limiting switch that turns on or off when the heating duration isset; switching means that controls supply of a supply voltage to aprimary side of the high-voltage transformer; and controlling meansincluding means for monitoring a phase of the supply voltage and meansfor judging whether or not a heating duration is set or not by detectingwhether the time-limiting switch is on or off, the controlling means, ifa heating duration is set, turning on the switching means according tothe phase of the supply voltage.
 11. The high-frequency heatingapparatus of claim 10, wherein, when an absolute value of the supplyvoltage is at a maximum, the controlling means turns the switching meansfrom off to on, and, when the supply voltage crosses a zero-Volt level,the controlling means turns the switching means from on to off.
 12. Thehigh-frequency heating apparatus of claim 11, further comprising:high-frequency heating output setting means that sets high-frequencyheating output, wherein the controlling means turns the switching meanson and off with a duty factor according to an output signal of thehigh-frequency heating output setting means, wherein the time-limitingswitch only controls supply of electric power to an electric circuitthat needs to operate as high-frequency heating is performed, whereinthe time-limiting device further includes a second time-limiting switchthat, at an end of the heating duration, turns from on to off with adelay relative to the time-limiting switch, and wherein the secondtime-limiting switch is connected in series with the switching means.13. The high-frequency heating apparatus of claim 10, wherein thecontrolling means turns the switching means on and off with apredetermined duty factor.
 14. The high-frequency heating apparatus ofclaim 12, further comprising: high-frequency heating output settingmeans that sets high-frequency heating output, wherein the controllingmeans varies the duty factor according to an output signal of thehigh-frequency heating output setting means.
 15. The high-frequencyheating apparatus of claim 10, wherein the time-limiting switch onlycontrols supply of electric power to an electric circuit that needs tooperate as high-frequency heating is performed.
 16. The high-frequencyheating apparatus of claim 10, wherein the time-limiting device furtherincludes a second time-limiting switch that, at an end of the heatingduration, turns from on to off with a delay relative to thetime-limiting switch, and wherein the second time-limiting switch isconnected in series with the switching means.